There is a fundamental gap in understanding how modular taper junctions behave in vivo, as indicated by the recent resurgence of problems with modular taper junctions in total hip arthroplasty (THA). Specifically, the topography of the trunnion and head taper surfaces, in the form of circumferential machining marks, is suspected to play a role. Continued existence of this gap represents an important problem because until it is filled, knowledge of how to improve modular junctions remains largely incomprehensible. The long term goal is to determine trunnion-head taper surface topography combinations that minimize micromotion in vivo and allow for the greatest forgiveness during assembly, thereby reducing the potential for fretting and corrosion. The overall objective of this application is to determine the relationship between surface topography and implant stability after assembly and cyclic loading and identify target best surface topographies for modern THAs. The central hypothesis is that surface topographies with shallower, more widely spaced machining marks will have higher pull-off loads and turn-off torques after assembly, less micromotion under cyclical loading, and less severe damage on retrieved implants. The rationale underlying the proposed research is that, determining the surface topography that minimizes micromotion will result in improved modular junctions, reducing implant failure. The central hypothesis will be tested under three specific aims: 1) Characterize trunnion- head taper surface topographies, global implant dimensions, and damage patterns of retrieved THAs; 2) Determine the factor most important for initial implant stability and later stability during cyclic loading by performin a parametric FEA of trunnion-head taper surface topography, load, implant global geometry, and material; and 3) Experimentally test both initial stability and stability under cyclic loading of trunnion-head taper topography combinations identified as ideal using FEA. Under aim 1, retrieval analysis will be used to identify ranges of implant characteristics consistent with grade of damage for evaluation with FEA. FEA will be used to achieve aim 2 to determine the surface topography that results in highest pull-off force and turn-off moment and lowest micromotion under cyclical loading. Experimental testing will be performed in aim 3 of the FEA identified best topographies. The approach is innovative because of the novel multi-scale FEA approach which links a global THA model to the local surface topography to determine how the local surface topography affects the entire implant. As a consequence, new strategies for reducing fretting and corrosion of modular taper junctions are expected to result. The proposed research is significant because it is the first step towards determining how to decrease fretting and corrosion in modular taper junctions. Ultimately, such knowledge has the potential to advance both FEA and experimental design and help reduce the growing burden of TKA revision surgery in the United States.